1. Field of the Invention
The invention relates to an improved heat exchanger and a method for making the improved heat exchanger. The heat exchanger may be, for example, a condenser, an evaporator, a cooling tower or a heat sink.
2. Description of the Related Art
Many types of heat exchangers may be found in the prior art. These heat exchangers function to transfer heat from one body to another. In this application, the term xe2x80x9cheat exchangerxe2x80x9d is defined as any device which exchanges heat from one fluid to another fluid, to a solid or to the environment. Condensers, evaporators, cooling towers and heat sinks are considered to be examples of such heat exchangers.
FIG. 1 illustrates an example of a prior art auger type ice making machine disclosed in U.S. Pat. No. 5,664,434 including an evaporator type heat exchanger. An auger type ice making machine 10 includes a cylindrical refrigerated casing 11 having a cooling pipe 14 wound around the outer periphery thereof. An auger 12 having a spiral blade 13 are disposed on a columnar main body 12A and mounted in the refrigerated casing 11 by being rotatably supported by bearings 20a and 20b. A shaft portion 12a of the auger 12 supported by the lower bearing 20a is coupled with the output shaft 17 of a drive motor 16 through a well-known spline coupling 18, while a bar-shaped cutter 22 is disposed at the upper end of an upper side shaft portion 12b supported by the upper bearing 20b. 
The lower bearing 20a is accommodated in an approximately cylindrical support member 19 capable of being mounted on the lower end of the refrigerated casing 11, while the upper bearing 20b is accommodated in a press head 21 mounted at the upper end of the refrigerated casing 11. Although not shown, the press head 21 includes a plurality of concave ice compressing passages each extending in an axial direction and flake-shaped ice passing therethrough is compressed and formed into ice columns. The ice columns discharged into a discharge cylinder 23 from the press head 21 are cut off by the cutter 22 to form flaked ice 24.
Further, the cooling pipe 14 is covered with a suitable heat insulating material 25. A water supply pipe 26 is connected to the lower end of the refrigerated casing 11 so that a fluid can flow therethrough and ice making water from a not shown ice making water tank is supplied in to the refrigerated casing 11 through the water supply pipe 26.
The combination of the cooling pipe 14 and refrigerated casing 11 may be considered the evaporator portion of the ice making machine 10. When the auger type ice making machine 10 is operated, ice making water is supplied from the water supply pipe 26 to a predetermined water level in the refrigerated casing 11, a not shown refrigerating unit is operated to cause a coolant (refrigerant, e.g.) to flow through the cooling pipe 14. The refrigerated casing 11 and cooling pipe 14 act to transfer heat from the water to refrigerant in the cooling pipe 14; heat from the ice making water flows through the refrigerated casing 11, to the cooling pipe 14 and is absorbed by the refrigerant in cooling pipe 14.
When the drive motor 16 is driven, the auger 12 is rotated through the output shaft 17 and the spline coupling 18, an ice layer made around the inner periphery of the refrigerated casing 11 is fed upward while being scratched or scraped by the spiral blade 13 and put into the not shown ice compressing passages of the press 21 and compressed therein so as to form ice columns. The ice columns discharged into the discharge cylinder 23 from the ice compressing passages are cut off by the cutter 22 rotating together with the auger 12 to form ice cubes 24 each having a suitable length.
Making the heat transfer from the ice making water to the refrigerant more efficient, leads to a more efficient ice making machine; heat from the ice making water is more easily removed with less energy. One prior art procedure to improve heat transfer creates spiral grooves on the inside of cooling pipe 14. Such a cooling pipe is known as rifled tubing, micro-finned tubing or inner-grooved tubing. This tubing has small ridges formed on its inside surface. These ridges may be created by forming a spiral groove on the inner surface of the tubing, thereby increasing the surface area of the inner surface of the tubing. The increased internal surface area reduces the liquid refrigerant film thickness which results in an increased effective temperature difference between the tube wall and the refrigerant gas-liquid interface, providing more heat transfer potential. The rifled tubing may also help promote annular flow, resulting in an increase in the amount of wetted surface area for evaporation. This type of tubing is disclosed in U.S. Pat. No. 4,660,630.
U.S. Pat. No. 4,660,630 also discusses modifications to the outside of a tube surface. The outside surface of a tube may be finned or knurled at some point in the manufacturing process to improve the efficiency of heat transfer tube.
However, while effective, rifled, finned, and knurled tubing is difficult to manufacture, and thus expensive. The additional cost of such tubing may not justify the increased efficiencies achieved by the rifled tubing. Further, when the use of tubing is not desired or required in a heat exchanger design, it can become even more complicated to decrease the thermal resistance of an irregularly shaped heat transfer surface by rifling, finning or knurling.
This invention is directed to an improved heat exchanger and a method for manufacturing an improved heat exchanger, where a surface of the heat exchanger has pits or bumps formed therein. These pits may be easily created by accelerating projectiles toward the surface of the heat exchanger and creating pits or bumps in a surface of the heat exchanger at portions where the projectiles impinge the surface of the heat exchanger.